CRM formula field help

CRM formula field help

Hello! i was hoping to get some help with a formula i'm creating within a module. 

I'm looking to make a formula that changes based on a date field but based upon the present date. This is the formula i have so far:

If(Now() < ${Instructors.Start Date}, 'Upcoming', If(Now() <= ${Instructors.Contract End Date}, 'Active', 'Completed'))

and the error i keep getting is this: You used the wrong type of data in an argument. Check what you entered between the parentheses of your functions to make sure you put the right kind of data in the right place.

I have it currently set to String for the formula return type, since it will be giving me words. But I can always change that if necessary. 

Thanks for any help you can give!